Black Death survivors and their descendants were healthier and longer-living, meaning that the plague served as a gigantic natural selection event eliminating many of the weak and frail from the population. ...
Elderly people and those who were in poor health to begin with were much likelier to die. To be sure, the plague kills healthy people, too. But, as with most infectious diseases, healthy people have a better chance at surviving. Ironically, the Europeans who made it through the Black Death inherited a much better world. Food prices dropped, labor wages increased, and there was a boost in the standard of living.
So they had that going for them. Lucky buggers.
